1. Field of the Invention
The present invention relates generally to steam turbine blades and, more specifically, to a two-lug side-entry turbine blade attachment for use with relatively small blades which are assembled into milled grooves.
2. Description of the Related Art
Turbine blades may be attached to turbine rotors in a variety of ways. One well known structure is the use of a "fir-tree" side-entry root. The root configuration derives its name from the fact that it employs at least two lugs which generally increase in size from lowermost to uppermost.
The basic fir-tree root configuration contains multiple potential load paths, with the magnitude of the resulting stresses therein dependent upon the precision of the initial fit between the root and its corresponding groove. These stresses are of particular concern for such potential failure mechanisms as high-cycle fatigue, low-cycle fatigue and stress corrosion cracking.
Blades with fir-tree roots are characteristically susceptible to important vibratory modes in which the neutral axis of vibration in the root is approximately parallel to the axis of the turbine rotor. For such vibratory behavior, the uppermost lands of a fir-tree root provide a large portion of the total root stiffness and load-carrying ability. For that reason, it is particularly important that these uppermost lands be in firm contact during turbine operation. Manufacturing tolerances must be selected so as to ensure that this firm contact occurs on the uppermost lands, while at the same time minimizing the peak stresses throughout the blade fastening structure.
To accomplish these ends, fir-tree roots are often designed with median tolerance dimensions which provide a very small clearance on the lower lands when the turbine is at standstill. The magnitude of this median lower land clearance is a function of the tolerances themselves. For a given fir-tree root design and application, larger tolerances require a larger median lower land clearance to ensure that the uppermost lands are in firm contact during turbine operation.
Certain characteristics tend to increase the magnitude of manufacturing tolerance deviations. One such characteristic is the use of different rotor diameters, root designs or number of blades per row in closely adjoining rows. Any of these features precludes the use of broaching as a groove manufacturing method and requires instead that intrinsically less precise milling machine methods be used. A related characteristic is the width of the lower lugs. Increased width raises the loads upon the milling cutter, thus decreasing the precision of its cutting path.
Certain characteristics of the blade, root, and groove also tend to increase the dimensional influence of manufacturing tolerance deviations. These include small absolute size, and relatively low applied steady loading.
Certain characteristics of the blade tend to increase the likelihood of adverse consequences due to imprecise fit of the root in its corresponding groove. One important such characteristic is a design in which the lowermost modes of vibration are untuned, in that they are permitted to be in resonance. Low modes tend to produce the largest high-cycle fatigue stresses in the root rather than elsewhere in the blade. Untuned blades are in general small in size relative to other blades in the same turbine.
Determining root and groove profiles with acceptable maximum and minimum clearances is extremely difficult, keeping in mind that zero clearance (surface to surface contact) must occur precisely at the lug or steeple lands when the centrifugal load is applied. For a two-lug side-entry turbine blade there are only two lands corresponding to the two lugs (there would be left and right lands disposed on opposite sides of the root center line, which is also the plane of symmetry, thus making a total of four lands, two at each lug).
Thus, a great deal of time and effort goes into designing each blade attachment for a steam turbine or combustion turbine. An example of prior art methods of designing side entry turbine blade roots is shown in U.S. Pat. No. 4,692,976, issued to Andrews. In that patent, a method is provided for producing a scalable two-lug (or tang) side-entry turbine blade with significantly reduced stress concentration attributable to centrifugal and bending loads on the blade root. The design incorporated therein equalizes the stresses at all points of stress concentration. As a result of the degree of precision which is required in the creation of the blade attachment, the surfaces of the blade root and groove are defined in terms of the lengths of their respective radii, the location of the pivot centers for the respective radii, the beginning and terminating points of each curved segment, and the length of the lands (or flats) associated with each of the two lugs.
In U.S. Pat. No. 4,824,328, issued to Pisz et al., another turbine blade attachment is disclosed in which the blade root and groove profiles are defined in terms of specific relationships.
A continuing need exists for a turbine blade attachment which reduces the magnitude of manufacturing tolerance deviations when the groove manufacture must be accomplished by milling. Also, a continuing need exists for turbine blade attachment which reduces the adverse consequences of manufacturing tolerance deviations, particularly with respect to high cycle fatigue and stress corrosion cracking.